Voting is still underway in the federal election, with polling booths set to shut at 6:00pm (local times).

A spokeswoman for the Electoral Commission says about 13.6 million voters will have cast their votes today at almost 8,000 polling booths across Australia.

Prime Minister John Howard and Labor leader Kevin Rudd voted this morning, each man taking the opportunity to hammer his message in an election poll which the latest polls say will be a cliffhanger.

The Newspoll in this morning's Weekend Australian put Labor one point ahead on the primary vote and with only a four-point margin after preferences.

Voting in his seat of Bennelong, Mr Howard repeated his message that the Coalition Government has done a good job running the country.

"We've got the men and women to deliver the best years yet for this nation," Mr Howard said.

"If you change the government you will change the fundamental direction of the country."

Mr Rudd cast his vote in his seat of Griffith, in the Brisbane suburb of Bulimba.

"I think we are going to do a more-than-competitive show," Mr Rudd said.

In other developments today, Federal Environment Minister Malcolm Turnbull said he would leave politics if he fails to be reelected to his Sydney seat of Wentworth.

Mr Turnbull, who holds Wentworth with a 2.5 per cent margin, put a full page ad in today's Sydney Morning Herald, warning voters they could face a by-election if his Labor opponent George Newhouse wins.

He has promised a legal challenge if Mr Newhouse wins but says if he eventually loses he will not stand for any other seat.

"If Mr Newhouse were to win and there was a by-election I'd obviously consider running again but the fact is that if I'm not elected in this seat I'm out of parliament and out of politics," he said.

Mr Turnbull is still disputing Mr Newhouse's eligibility to stand in this election, with the Labor man saying there has been a campaign of dirty tricks against him.

"Malcolm is just beating up a scare campaign because he can't compete on policy issues," he said.